It has become conventional to employ glass laminates instead of glass for such applications as window panes, windshields for automobiles and other motor vehicles and the like. Since glass breaks easily and shatters while breaking, it presents a substantial risk of serious injury.
Many types of laminated glass are known in the prior art. One type includes two or more sheets of glass bonded together by means of an elastomeric interlayer material. Polyvinyl butyral is commonly used as the interlayer because of its adhesion to glass when subjected to suitable termperatures and pressure, because it is transparent when bounded between two sheets of glass, and because of its resistance to elongation whenever an object impacts such a laminated glazing. Other interlayer materials employed include polyurethane compositions which are sometimes introduced as partial polymers and sometimes cast in place.
Another type of glass laminate is the combination of a polycarbonate and glass. The polycarbonates are especially advantageous because of their high rigidity, high impact strength and stability under load at temperatures from well below -100.degree. C. to as high as 150.degree. C. and above. Additionally, there is very little light absorption by the polycarbonate layer of a safety pane coupled with high ultraviolet light and infrared radiation absorptions. One difficulty in polycarbonate laminates arises from the fact that the glass and polycarbonate have different coefficients of thermal expansion and therefore expand at different rates. It is suggested in U.S. Pat. No. 3,821,071 to mount the polycarbonate and glass sheets slidably against one another in face-to-face relationship in a frame. While this arrangement overcomes some of the problems, it is not entirely satisfactory.
Accordingly, it is the object of this invention to provide a new glass unit which includes one or more polycarbonate layers and which avoids the problems inherent in the differential heat expansion rates of glass and the plastic material. This and other objects of the invention will become apparent to those skilled in the art from following detailed description in which the sole figure is a cross-section of a glass unit constructed in accordance with this invention.